The present invention relates to a method of forming a resist pattern.
In immersion exposure, using a system of causing the space between a lens and a face to be irradiated (wafer) to have a high refraction index by forming a water film (meniscus) at a minute gap between the lens and the wafer while utilizing the surface tension of water, an effective lens numerical aperture (NA) can be enhanced up to the vicinity of the water refraction index 1.44, as compared with usual dry exposure, and industrially practical application of the exposure is progressing as a technique that enables the resolution limit of lithography to be miniaturized.
In the immersion lithography, since immersion water lies between the lens and the resist being an uppermost layer film of the surface to be irradiated (wafer), there is such anxiety that the use of a chemical amplification resist for usual dry exposure allows a low molecular weight content such as a photoacid generator or a base therein to dissolve in the immersion water to cause, consequently, the pollution of an exposure device such as the lens. Further, when moving the water film (meniscus) over the wafer, if the resist has a low hydrophobicity, a micro droplet remains on the resist after the movement of the meniscus, which induces pattern defect caused by the influence of longtime local contact to moisture. Therefore, a technique, in which a resist upper layer protective film (a topcoat) that dissolves in a developer for not allowing the immersion water to directly contact the resist is used over the resist, is introduced. Furthermore, a topcoat-less resist, in which a small amount of a polymer having a small critical surface tension (mainly, a fluorine-containing polymer) is mixed in the resist liquid as a hydrophobic additive and the hydrophobic additive is concentrated to the surface alone while utilizing the surface segregation effect of the hydrophobic additive at the time of forming a coating film to spontaneously form the two layers of the resist and topcoat as a single coating film, has been developed and offered commercially.
In the immersion lithography, as shown in FIG. 1, the immersion water is formed as a meniscus in a film shape on the photosensitive resist layer, and scanning exposure is practiced by scanning the wafer (in the arrow A direction in FIG. 1) while irradiating light through the meniscus. On this occasion, in order to enable prevention of the dissolution of a low molecular weight compound such as a photoacid generator or a base in the chemical amplification resist into the immersion water, and to allow high-speed and smooth meniscus movement without leaving a droplet, techniques for bringing the surface of the resist layer to be a state in which elution hardly occurs and to have a hydrophobicity are introduced.
One of these techniques is the topcoat process, in which the resist layer is constituted of a two-layer film formed by two coating treatments and a resist upper layer protective film (the topcoat) is coated on a usual resist. The topcoat is frequently constituted of a fluorine-containing polymer having an alkaline developer solubility, in order to be a film that is soluble in an alkaline developer to be automatically peeled off at the time of the development and has a high hydrophobicity. FIG. 2(a) shows the process flow in the topcoat process. However, there are such problems that the topcoat process is lengthy because it is accompanied with two coating treatments, and that the cost of materials is high because two kinds of chemical solutions for the photosensitive resist and the topcoat are used. In addition, since that the topcoat contains a developer soluble group and that it has a high hydrophobicity are in a trade-off relation, frequently it cannot be said that the hydrophobicity is sufficient for higher throughput exposure by more high-speed scan.
On the other hand, as another technique, the topcoat-less resist, in which a small amount of a polymer having a small critical surface tension (a fluorine-containing polymer) is mixed in a photosensitive resist liquid as a hydrophobic additive and the hydrophobic additive is concentrated (segregated) to the surface alone while utilizing the surface segregation effect of the hydrophobic additive at the time of forming a coating film to spontaneously form a segregated layer having the effect of the photosensitive resist and the topcoat as a single coating film by one coating treatment, has been developed and commercialized in the market. The material can suppress the lengthy property of the topcoat process, and, consequently, the increase in the cost of equipment (one coating cup, one bake plate) and the increase in the cost of materials.
FIG. 2(b) shows a process flow using the topcoat-less resist material. As a hydrophobic additive to be added to the topcoat-less resist material in a small amount, when a hydrophobic additive is used which is soluble in a developer as is the case for the topcoat material, the use of a hydrophobic additive having no alkali solubility in an extremely small amount may be considered. Further, there is also such case that a hydrophobic additive, which itself is deprotected by the catalytic reaction of an acid generated by the exposure to change into a state of being soluble in an alkaline developer only in the exposed portion at the time of subsequent post-exposure bake (PEB) as is the case for the chemical amplification resist, is used. As described above, there may be mainly three constitutions as the hydrophobic additive.
When the hydrophobic additive to be added to the topcoat-less resist material is an alkali soluble hydrophobic additive, defects in the resist pattern hardly generate, because the hydrophobic additive is dissolved at the time of the alkali development and, therefore, particles adhering to the resist surface are removed simultaneously. Further, after the removal of the hydrophobic additive, the resist has the hydrophobicity of itself alone and, as a result, is more hydrophilized than before the alkali development. Therefore, there is also a low probability of leaving a particulate residue 10d defect (blob) as shown in FIG. 11(c) caused by a too high hydrophobicity at the time of deionized water rinse after alkali immersion not to allow the resist to be washed cleanly. However, the hydrophobic additive tends to have a low segregation property because an alkali developing property is to be given, and, as is the case for the problem in the topcoat, frequently a sufficient hydrophobicity cannot be obtained.
When a hydrophobic additive having completely no alkali solubility is used as the hydrophobic additive, the hydrophobic additive remains as an insoluble matter at the time of the development, and there are such risks as causing a residue 10b defect as shown in FIG. 11(a), inducing a micro bridge 10c as shown in FIG. 11(b) even in a minor instance, or leaving a particulate residue defect (blob) (FIG. 11(c)) due to a too high hydrophobicity at the time of the deionized water rinse after the immersion into an alkali.
Further, even when a hydrophobic additive is used that itself is deprotected by an acid to change into a hydrophobic additive soluble in an alkaline developer in an exposed portion alone at the time of subsequent post-exposure bake (PEB), the generation of defect is considered to be generated in unexposed portions, as is the problem at the time of using the hydrophobic additive having completely no alkali solubility. That is, in such processes as a hole process that uses a positive resist or a process that uses a dark field mask such as the process of forming a trench for Cu wiring, it can be said that these defect risks are high because the background is left as an unexposed portion.
In conventional immersion lithography processes, the process using the topcoat is widely employed for realizing the immersion exposure while maintaining the performance of the chemical amplified resist. However, there are such problems that the cost of equipment and materials becomes high, and that the hydrophobicity can hardly be obtained for high-speed scan. On the other hand, in processes employing the topcoat-less resist for aiming at the cost reduction and the suppression of the resist pattern defect in the immersion lithography process, there is the same problem as that in the topcoat process (the hydrophobicity for high-speed scan) in the instance of an alkali soluble hydrophobic additive, and a problem of the pattern defect caused by insolubility in a developer in the instance of an alkali insoluble hydrophobic additive or a hydrophobic additive of a type that changes polarity by exposure. These pattern defects cause such problems that a hydrophobic additive re-adheres onto the resist pattern, and that defects caused by the immersion increase when the resist surface layer of unexposed portions maintains high hydrophobicity. These problems appear more remarkably when forming a hole pattern that leaves much resist surface layer.
In Japanese Patent Laid-Open No. 2007-180253 (Patent Document 1), in the process employing the topcoat, defects caused by immersion are suppressed by peeling off a protective film with a peeling liquid capable of selectively dissolving the protective film (the topcoat) over the substrate after the immersion exposure process.
However, according to the method in Patent Document 1, it is necessary to change the content of the peeling liquid on a case-by-case basis in accordance with the material property of the protective film to be used. Further, it is necessary to add a peel-off process with the peeling liquid, which leads to the increase in the product cost and the lowering of the production efficiency. Further, when it is used for the top-coat/resist process, since no distinct interface exists between the hydrophobic additive layer and the resist layer, sufficient peeling and removal of the hydrophobic additive are difficult and defects caused by the immersion cannot be suppressed.
Japanese Patent Laid-Open No. 4-217258 (Patent Document 2) discloses a method of forming a resist pattern, in which the resist surface is modified without changing a developing apparatus to enhance the hydrophilicity of the resist. According to the method of forming a resist pattern of Patent Document 2, after performing the exposure to the resist, ozone generated by heating and irradiating ultraviolet rays is reacted with the resist to impart the hydrophilicity to the resist. For example, a methyl group of the resist made of novolac resin is converted to aldehyde by an ozone treatment to enhance the hydrophilicity of the resist surface.
However, according to the method described in Patent Document 2, after exposing and projecting a reticle pattern to the resist, ultraviolet rays are irradiated, and, therefore, the shape of the formed resist pattern is occasionally softened or deformed by the ultraviolet irradiation after the exposure. Particularly, when the resist pattern includes a minute wiring and a hole having a size of 100 nm or less, the softening and deformation become more remarkable.
Japanese Patent Laid-Open No. 2000-89475 (Patent Document 3) discloses a method of forming a resist pattern, in which the surface of the resist is modified by irradiating light to enhance the hydrophilicity of the resist. According to the method of forming a resist pattern of Patent Document 3, after coating the resist and before exposing and projecting the reticle pattern, light is irradiated to the resist to raise the hydrophilicity of the resist and to enhance the affinity between the resist and the developer, thereby lowering the development defect.
However, according to the method described in Patent Document 3, the resist surface is subjected to a hydrophilizing treatment before the exposure and projection treatment. Accordingly, the hydrophobicity of the resist surface, which is necessary for the scanning exposure in the immersion lithography, is lost prior to the exposure, and it becomes difficult to exert the primary purpose of the topcoat-less resist.